Positron emission tomography (PET) is one of the important methods of nuclear medicine imaging. It is widely used in other fields such as oncology and neurology, and plays a very important role in medical research and clinical diagnosis. PET can observe the cellular metabolic activity at the molecular level and provide an effective basis for the early diagnosis and prevention of the disease. In the process of PET imaging, the tracer decays in the tissues of the human body. The positrons generated during the decay react annihilate with negative electrons to generate a pair of gamma photons with energy of 511 kev in opposite directions. The photons are recorded by the ring probe to generate projection data. The inversion of projection data by some mathematical methods can reconstruct the spatial concentration distribution of tracer in the human body.
In order to obtain more patient's physiological information, mixed tracers imaging has become a hot topic in PET imaging research. Traditional methods require individual injections, independent scans, and independent imaging for each tracer, which undoubtedly increases the patient's time, expense, and safety concerns. Therefore, double injection-single scan is a more reasonable technique for processing PET mixed tracer imaging. However, since each tracer decay produces gamma photons with energy of 511 keV, it is impossible to directly separate the counting photons generated by each tracer in hardware.
Traditionally, the methods of direct fitting and kinetic parameter estimation are often used for the image reconstruction of mixed tracers PET concentration distribution. The former method mainly uses some mathematical methods to directly fit the time activity curve (TAC). Although the calculation speed is fast, the reconstructed image quality is poor. The latter approach incorporates a compartment model and is computationally complex by estimating the k parameters to fit the TAC curve. In addition, these algorithms require an interval (10-20 minutes) between two tracer injection times, and the TAC curve for the second tracer can be obtained by analyzing and calculating the TAC curve for first tracer. In other words, these algorithms are not suitable for reconstructing PET concentration distribution image in the case of simultaneous injection.